The invention relates to a process for the removal of hydrogen from hydrogen compounds dissolved in silicon tetrachloride or germanium tetrachloride by chlorination of the hydrogen compounds.
In the production of optical fibres for transmission of information on a basis of SiO.sub.2 /GeO.sub.2 glass, OH-impurities play an important part because of their optical absorption. For high-quality fibres these impurities have to be reduced to a very low level, namely below 0.1 ppm by weight (corresponding approximately to 4.5 dB/km additional attenuation at a light wavelength of 1.38 .mu.m).
The chief sources of OH-impurities in optical fibres are hydrogen comprising impurities in the starting materials SiCl.sub.4 and GeCl.sub.4. If, for example, the PCVD process is used for production of the fibres, approximately one-eightieth of the H atoms supplied in the gas flow (SiCl.sub.4 +GeCl.sub.4 +O.sub.2) are incorporated into the glass as OH-groups (M. Lennartz, H. Rau, Betty Trafford, J. Ungelenk; ECOC 83-9th European Conference on Optical Communication (1983) 21-24). Total removal of such hydrogen impurities from the starting material is thus a technically important necessity.
It is known from (German patent) DE-AS No. 12 63 730 that germanium tetrachloride can be purified with hydrochloric acid and chlorine. In this process arsenic and other similar impurities are removed.
According to DE-AS No. 19 48 911 spontaneously or readily flammable hydrogen silanes can be removed from trichlorosilane, silicon tetrachloride or mixtures of these by adding anhydrous chlorine at a temperature in the range from -30.degree. to +300.degree. C. in a quantity corresponding to one to two times the quantity which is stoichiometrically required for conversion of the silicon-hydrogen bond to be chlorinated. The reaction also occurs in the desired manner, below and above that temperature range, either in the dark or during exposure to illumination, e.g. with UV light. Light thus has little appreciable effect on the course of the process.
With the process known from DE-AS No. 19 48 911 a selective partial chlorination of hydrogen silanes hydrogenated beyond trichlorosilane is obtained, i.e. trichlorosilane is not chlorinated. This process is therefore unsuitable for removing all the hydrogen from the hydrogenous compounds dissolved in silicon tetrachloride.
It is known from DE-OS No. 28 05 824 how to purify silicon tetrachloride containing as an impurity a silane compound which has in its molecule at least one hydrogen atom bound directly to the silicon atom by irradiating the silicon tetrachloride with UV light in the presence of chlorine. Trichlorosilane is thus also chlorinated in this manner. Removal of the resultant hydrogen chloride presents no problems (H. Rau, J. Chem. Thermodynamics 14 (1982) 77-82). According to DE-OS No. 31 35 916 hydrocarbon compounds and other hydrogen impurities are also removed by the ultraviolet chlorination method known from DE-OS No. 28 05 824. Impurities of this kind can find their way into the silicon tetrachloride in the course of previous purification measures. Thus it is known from, for example, DE-PS No. 867 544 that liquid chlorides of elements in group 4 of the periodic table can be purified with the aid of organic compounds, e.g. alcohols.
In the course of the research which led to the invention, however, it was established that the reaction with chlorine accompanied by UV radiation does not lead in every case to a total removal of hydrogen; for example, ethyl alcohol, C.sub.2 H.sub.5 OH, reacts with chlorine to form chloral (Holleman-Richter, Lehrbuch der organischen Chemie, 37th-41st edition (Berlin 1961), p. 230): EQU C.sub.2 H.sub.5 OH+4Cl.sub.2 =CCl.sub.3 CHO+5HCl
in which an H atom is left behind.